The bones inside your body make a hard frame called the skeleton, and that frame is always being rebuilt. Bone remodeling begins before birth and continues until death, and old bone is constantly broken down and replaced by new bone, and the remodeling is a four-step cycle: activation, resorption, reversal, then formation. Osteoclasts are the only cells known to be able to break down (resorb) bone, while bone-building cells (osteoblasts) and bone-removing cells (osteoclasts) balance each other to keep bone healthy, and in each remodeling cycle, osteoclasts take only about 2 to 4 weeks to remove old bone. Osteoblasts that become buried in bone matrix turn into osteocytes, which are linked to each other by a network of tiny channels. Compact bone is built from osteons: a central canal with blood vessels surrounded by rings of matrix, with bone cells in small spaces between them. Lamellar bone is strong because collagen fibrils are laid down in alternating directions, similar to plywood, and bone mineral gives bone rigidity and load-bearing strength, while the organic matrix gives it elasticity and flexibility.
Spongy bone's trabeculae follow the lines of stress and can realign if the direction of stress changes. Wolff's law describes the observation that long bones change shape to accommodate the stresses placed on them, and bone strength depends on bone mass, geometry and composition, material properties and microstructure. Bones get stronger when you use them by being active, and even after bones stop growing longer, they can keep getting thicker throughout life when muscle activity or weight puts more stress on them. In prolonged weightlessness, bone mass appears to fall because the lack of stress slows the formation of osteoblast cells. In a study on the International Space Station (ISS), hip bone strength fell by an average of 2.5% for each month of flight, and crew lost spongy bone in the hip at 2.2 to 2.7% and dense outer bone at 1.6 to 1.7% for each month in space, on average.
Doctors see and mend bones in several ways. Doctors use X-rays to see whether a bone is broken, and bones show up white on an X-ray because they absorb the most radiation. A DXA scan uses low-dose X-rays to measure how much calcium and other minerals are in a bone, usually at the hip and spine. Wilhelm Röntgen found X-rays by accident in 1895 while experimenting with vacuum tubes, and his X-ray picture of his wife's hand showed the bones of her hand and her ring. A cast or splint holds a broken bone still while it heals, and children's broken bones can heal well and heal quickly. Broken bones heal in three stages: inflammatory, reparative and remodeling.
By the end of the eighth week after conception, the skeleton's pattern is laid out in cartilage and soft membranes, and bone formation begins. Most bones form when a cartilage model is replaced by bone, a process called endochondral ossification, but some bones form inside sheet-like membranes instead of from a cartilage model, a process called intramembranous ossification. A baby's skull bones are not firmly joined at birth, which leaves soft spots on the head. The fontanelle at the back of a baby's head usually closes by age 1 to 2 months, and the one at the top within 7 to 19 months. Baby skull bones are joined by sutures that gradually gain minerals and harden, a process called ossification. A skull's cranial seams close at different times, with the first (the metopic suture) closing at about 3 to 9 months and others not until adulthood.
A long bone has a hollow shaft (diaphysis), flared ends called metaphyses below the growth plates, and rounded ends called epiphyses above them. Long bones grow longer at growth plates near their ends, and cartilage in the growth plate keeps growing until it stops in the early twenties, when the plate turns to bone and the bone can no longer grow longer. Yellow marrow is found in the large cavities of large bones and is mostly fat cells. In infants red marrow fills the bone cavities, but in adults it is mostly limited to the skull, ribs, sternum, collarbones, vertebrae and pelvis. Soft bone marrow inside many bones makes most of your blood cells, and red blood cells are made in the red marrow inside bones.
Of the 206 bones in an adult skeleton, 80 are axial (skull, hyoid, ear bones, spine, ribs and breastbone) and 126 are appendicular (arms, shoulders, hands, legs, hips and feet). A skull has 8 bones in the part that holds the brain and 14 bones in the face, and the femur, or thighbone, is the longest and strongest bone in the human body. Skeletons differ from animal to animal, and not everything stiff in your body is bone. The part of your ear you can see is made of cartilage, a tough bendy tissue, not bone, and a shark has a skeleton made of cartilage instead of bone. Giraffe necks have seven big bones, the same as almost all mammals, while the necks of sauropod dinosaurs had up to 19. A bird's bones are hollow and light, which helps it fly. Bird bones have thin walls and are filled with extensions of the lungs' air sacs, yet resist the bending stresses of flight, and T. rex bones held large air spaces linked to air sacs, and more than 10% of its body volume may have been air.
Old bones can answer questions about the past. The femur is one of the best-preserved bones in skeletal remains, so it contributes a great deal to archaeology and forensic science. Archaeologists can tell something about a person's work from bones, because the pull of a laborer's muscles reshapes the bone where the muscles attach. Lucy, a famous early human relative found in Ethiopia, has a skeleton that is about 40% complete, and she is about 3.2 million years old. She got her nickname from a Beatles song that was playing in the team's camp. Lucy's bones had no duplicates, which supports the conclusion that they came from one individual. Her small skull, long arms and cone-shaped rib cage resemble an ape's, but her spine, pelvis and knee are more human-like because she walked upright.
Words to know
- osteocytes: buried bone cells
- resorption: bone breakdown
- trabeculae: spongy bone struts
- fontanelle: baby's soft spot
- lamellar: layered type
